This invention relates to a video clamp circuit. More particularly, the present invention relates to a video clamp circuit which reduces the clamp performance of a switching circuit for pedestal clamp in accordance with the magnitude of a noise level contained in the video signal and removes an energy diffusion signal of a broadcasting satellite signal by means of a clamp pulse signal which is synchronized at a position subsequent to a burst signal provided to a horizontal synch pulse signal when the noise level is low.
In transmitting a video signal using a broadcasting satellite (or transmission via satellite), a video signal is subjected to frequency modulation (FM), and an energy diffusion signal in the form of a triangular wave signal of about 30 Hz is superposed with the video signal as shown in FIG. 1(A) in order to prevent interference at the time of demodulation.
Accordingly, a television receiver for receiving such a video signal is generally equipped with a clamp circuit subsequent to the demodulation circuit in order to clamp the pedestal level in synchronism with a synch signal and to remove the triangular wave signal (energy diffusion signal). FIG. 1(B) is a partially enlarged view of FIG. 1(A) and illustrates the pedestal clamp position while FIG. 1(C) is a partially enlarged view of FIG. 1(B) and is useful for explaining the time relationship of the clamp pulse.
In accordance with a conventional system, as shown in FIG. 2, a down-link signal from the satellite, which is frequency-converted into a video intermediate frequency (IF) of 70 MHz bandwidth, for example, is demodulated by an FM demodulation circuit 1, is then amplified by a first video amplification circuit 2 and is delivered to a de-emphasis circuit 3 for returning the signal back to its original form prior to the pre-emphasis placed on the signal when transmitted in order to prevent deterioration of the S/N ratio in the high bandwidths used. The signal is further amplified by a second video amplification circuit 4. An intercarrier system has generally been employed for the audio signals which system amplifies a composite video signal obtained by synthesizing the video signal and the audio carrier by the same intermediate frequency amplification circuit. A part of the output of the abovementioned second video amplification circuit 4 is sent to an audio demodulation circuit not shown so that the audio carrier is demodulated to provide the audio signal.
On the other hand, the output of the second video amplification circuit 4 is delivered to a video filter 5 which blocks the passage of the audio carrier but allows the passage of only the video carrier, and is then applied to a switching circuit for pedestal clamp consisting of an FET switching circuit 6, for example. The output of this FET switching circuit 6 is applied to a third video amplification circuit 7, and a part of the output of the third video amplification circuit 7 is in turn applied to a synch amplification circuit 8 which amplifies a synch signal separated from the input video signal and applies it to a logic circuit 9 which generates a clamp pulse in order to obtain a predetermined clamp level at a pedestal clamp position subsequent to a color burst signal that is inserted to the back porch of the horizontal synch signal as shown in FIG. 1(C). The logic circuit 9 generates the pulse for pedestal clamp subsequent to the color burst signal to be inserted to the back porch of the horizontal synch signal for each line of the field in the time relationship as shown in FIG. 1(C) thereby to actuate the FET switching circuit 6 and clamp the video signal sent from the video filter 5 at a predetermined position and thus remove the undesired triangular wave signal (the energy diffusion signal) of about 30 Hz. The system which arranges the pedestal levels in this manner is referred to as the "pedestal clamp system".
In the pedestal clamp systems in general, the clamp performance as a characteristic feature of the pulse clamp is extremely strong when the S/N ratio is good and a removing ratio of the diffusion signal as high as 46 to 50 dB, for example, can be obtained by the known systems. When the S/N ratio is not too good, on the other hand, since clamp is applied to the signal superposed with the noise, the pedestal level fluctuates in proportion to the level of superposition of the noise so that thin random horizontal stripes occur on the field or the picture surface. In a range in which the S/N ratio is not so bad and the picture can tolerably be observed, it is not possible for the logic circuit 9 to deliver the clamp pulse to the FET switching circuit 6 with accurate timing so that the clamp pulse is produced at a position other than the predetermined pedestal clamp position, such as inside the video signal for example, and a large amount of noise is produced on the field or the picture surface. In other words, the S/N level in such a case does not provide a tolerable picture at all. These are represented by dash lines (ii) in FIGS. 7(A) and 7(B).
FIG. 3 shows an example of the circuit construction of a so-called "reverse phase superposition system" which arranges the pedestal levels to a predetermined level. Reference numerals 1 through 5, 7 and 8 correspond to those used in FIG. 2. The output of the video filter 5 is applied to one input terminal of a differential amplification circuit 51, while a part of the output of the third video amplification circuit 7 is applied to a sample-and-hold circuit 52 and to the synch amplification circuit 8. After the synch signal is amplified by the synch amplification circuit 8, waveform shaping is effected in a waveform shaping circuit 53, thereby actuating the abovementioned sample-and-hold circuit 52.
The sample-and-hold circuit 52 samples and holds a d.c. voltage level following the color burst signal to be inserted to the back porch of the horizontal synch signal for each line of the field, and the signal is sent to an integration circuit 54 to produce the triangular wave signal and is then input to an amplification circuit 55, where the triangular wave signal is adjusted to a level of an opposite phase to the tilt of the pedestal level of the video signal applied from the video filter 5 to the differential amplification circuit 51, and is applied to the other input terminal of the differential amplification circuit 51. In this manner, the differential amplification circuit 51 removes the energy diffusion signal from the video signal sent from the video filter 5.
In this reverse phase wave superposition system, the triangular wave signal (or the energy diffusion signal) is detected and is then applied after being adjusted in the reverse phase in order to offset the triangular wave signal and to arrange the pedestal levels. Accordingly, though the erroneous operation due to noise is less, erroneous operation would occur if the level, which is to be applied after the phase inversion, fluctuates so that the removing ratio of the diffusion signal becomes lowered. Furthermore, due to the temperature stability of the circuit or its variance with the passage of time, 30 Hz stripes would occur on the field or the picture surface when the abovementioned removing ratio of the diffusion signal becomes below 30 dB. These are illustrated by dot-and-chain line (iii) in FIGS. 7(A) and 7(B).